The motive for the murder of Jill Dando may be hard for any rational person to imagine.

But her death has disturbing echoes of other cases where celebrities have become victims of obsessive fans.

On both sides of the Atlantic some famous names have faced unwanted attention, leading to persistent harassment, threats of violence and in some cases death.

In the United States the phenomenon has become known as "celebrity stalking".

Fascination

To many people the lives of the rich and famous are a source of fascination. But many stars resent the constant intrusion into their lives, and crave privacy.

Madonna: pursued by stalkers

Such attention may be irritating, but is mostly harmless. Dealing with fan mail and autograph hunters is the price of celebrity.

But the obsessive fan wants more. Some become convinced they have a relationship with the target of their admiration.

But when adoration turns to anger, perhaps because they feel rejected by the star, the delusions of the obsessive fan can become dangerous.

Madonna, Brad Pitt, Cher, Sylvester Stallone, Kevin Costner, and Linda Ronstadt are all reported to have suffered from the unwanted attention of admirers.

Obsession

The film director Steven Spielberg spoke of his fears of being killed after a "sexually obsessed" stalker attempted to break into his home.

Types of stalking

Intimate: the stalker and victim know each other; motive is either revenge for ending relationship, or an attempt to get back together

Obsessed fan syndrome: stalker has never met the victim; may be suffering from a mental disorder

Erotomania: stalker deludes himself into thinking the victim loves him, even though they have never met

The man, who was carrying handcuffs and a knife, was jailed for 25 years.

In Los Angeles a young actress, Rebecca Schaeffer, was shot dead at her home.

The killer, Robert Baldo, said he had chosen his victim because of her fame. He said he felt he knew her because he had seen her on television.

Deputy district attorney, Marcia Clark, who prosecuted Baldo, summed up his motives.

She said: "He chose the easy way to the top.

"The only way he knew of to get fame was to kill someone famous and attach himself in a parasite-like fashion to the fame that the person had."

Television

Experts in the US believe television is a significant factor in the increasing number of attacks on people in the public eye. The box in the corner brings brings famous faces into the homes of the lonely and disturbed.

In fear: Steven Spielberg

The object of the obsession may unwittingly reinforce the fantasies of the delusional fan by showing them kindness in stopping to talk to them, or personally answering fan mail.

According to one theory, the "nicer" and more approachable a celebrity appears, the greater is the likelihood that they will face this kind of attention.

Sometimes, the stalker will inflict violence on others in an attempt to impress the person they admire. John Hinckley tried to assassinate Ronald Reagan because of his obsession with actress Jodie Foster. Tennis star Monica Seles was stabbed by a fan of Steffi Graf.

In the UK, the growth of stalking has led to the introduction of the Protection from Harassment Act, which became law four years ago.

Terror

Scotland Yard now issues guidance for officers dealing with stalking cases. Detective Inspector Hamish Brown is no doubt about the impact on the person being pursued.

"I have seen at first hand the terrible distress to victims that stalking and harassment can cause," he says.

Terrified: Sarah Lockett

Two years ago, a 30-year-old man was jailed after stalking Meridian Television presenter Sarah Lockett. He wrote dozens of letters to her, and waited outside the studios. He tried without success to find out her home address.

He told his victim: "Every celebrity needs a stalker."

Ms Lockett said she was terrified; the unwelcome attention by her "fan" had left her unable to sleep at night, and made it difficult for her to continue working.

Guidance

The way in which news broadcasters are increasingly becoming victims of stalking has been noted with concern in the United States.

Scotland Yard's advice to victims

Carry a mobile phone and personal attack alarm

Alter your daily routines

Keep calm, show no emotion

Keep a record of what happens

Screen phone calls, do not respond to letters

Do not agree to a meeting

Because of the nature of their job, they are more accessible to the public than movie stars and rock musicians, who are inevitably surrounded by tight security.

A recent survey showed that threats to US radio and television stations by individuals with severe mental illnesses were "relatively common".

At a recent conference, US broadcasters were given the following advice on dealing with stalkers.

Do not assume that the senders of threatening letters are more likely to approach you than those professing love. Take both equally seriously.

Do not return gifts and letters; save them for the police.

Do not eat any food sent by a stranger.

Do not reveal personal details about broadcasters on the air.

Use bullet-proof glass to protect on-air staff who are visible from outside the station.

The dilemma now facing all broadcasting organizations is how to remain accessible to the public they serve, while at the same time protecting their celebrity presenters from the tiny minority of people who might do them harm.

Such was Jill Dando's popularity that her friends and colleagues thought she did not have an enemy in the world.

Sadly, it is an assumption no-one in the public eye can now afford to make.